Nitric acid concentration



y 1940- R. F. PETERSON El AL 2,201,631

nmuc ACID coucsmaynou Filed May 14, 1938 2 Sheets-Sheet 1 1mummam:"

' Ha! I? i'Tpezenron lgp i-g INVENTORS A TTORNEY May 21; 1940- R. F.PETERSON ET AL NITRIC ACID CONCENTiRATION Filed May 14, 1938 2Sheets-Sheet 2 Kala/2 ffPezem oh By ATTORNEY Patented May 21, 1940UNITED STATES PATENT OFFICE NITRIC ACID CONCENTRATION Application May14, 1938, Serial No. .2.ll'7,89l i 2 Claims, (01. 23160) This inventionrelates to a process for concentrating nitric acid, and moreparticularly to an improved continuous process and apparatus forcarrying out the step of denitrating residual solutions of dehydratingagents in the concentration of nitric acid.

This case is a continuation-in-part of our copending application, SerialNo. 738,948, filed August 8, 1934.

The concentration of weak nitric acid is an important industrialprocess, since substantially anhydrous mixed acids are required for manynitration operations, while the spent nitrating acids are relatively lowin nitric acid content. For the concentration of such acids the use of adehydrating agent is necessary. Sulfuric acid has been the most commonlyused agent for this purpose, though other materials have been suggested,such as phosphoric acid, arsenic acid, calcium nitrate, and the like.

Because of its excellent dehydrating properties and its economicavailability, sulfuric acid has been the agent employed in substantiallyall methods in practical use for the concentration of nitric acid. Themagnitude of such concentration operations has been so great thatcontinual efforts have been put forth to bring about improvements in theprocess. Among the methods proposed with this object in view have beenthose of Pauling, in U; S. Patent 1,031,865; Kaltenbach (French Patent594,865); Davis, Handforth and Kirst (U. '5. Patent 1,922,278);Handforth (U. S. Patent 1,922,289); Zeisberg (U. S. Patent 1,928,749);and many others. In the various methods of the art many disadvantagesexist, such as inefficiency through introduction of steam which mustlater be removed, complicated apparatus requirements, the necessity forhigh-pressure steam for evaporation purposes, failure to bring aboutsatisfactory heat exchange in the concentration apparatus, and manyothers.

The object of our invention is an improved process for the concentrationof dilute nitric acid, utilizing a dehydrating agent. A further objectis such a process in which there is no dilution of the dehydrating agentby the introduction of water vapor into the process from externalsources. A still further object is a process in which the maximum heatexchange effect is obtained. A still further object is an improved andmore efiicient apparatus for carrying out our process. Additionalobjects will be disclosed as the invention is described more fullyhereinafter.

We have found that the foregoing objects are attained and disadvantagesovercome by means of our invention, which comprises causing thesubstantially spent dehydrating agent containing nitric acid to fiowin acondition of high surface exposure, for example, in the form of 'a-thinlayer or film, down over the inner surface of a substantially verticaltube which is heated externally.

Although the vertical evaporating tubes by themselves are sufiicient forbringing about the production of a concentrated nitric acid, if a largeenough quantity of dehydrating agent is used and very high temperatures,we prefer to employ the tubes in conjunction with a preliminarydehydrating tower or vessel, thereby making economical operationpossible. Preferably.v also, a second fractionating or denitrating towerwill follow the vertical boiling tubes, and the last traces of nitricacid will be removed in this second tower. The nitric vapors given offfrom the second fractionating tower, and from the vertical boilingtubes, desirably are returned to the dehydrating tower. The return ofthe nitric acid vapors will preferably be through the vertical tubes,though separate vapor returning devices may be provided if desired. Animportant feature, also, is the fact that in the most preferredembodiment the dehydrating tower is connected to more than one verticalboiling tube, so that the solution for denitration passes in parallelflow through the various vertical tubes. If a second fractionating toweris employed, these tubes, adapted for parallel flow, serve to connectthe dehydrating and the denitrating towers.

In carrying out our process in its preferred form, we introduce amixture of dilute nitric acid and a dehydrating agent into the top of adehydrating tower. This mixture becomes "heated by the hot vaporsentering at the bottom, and concentrated nitric acid leaves at. the topof the tower. The diluted dehydrating agent leaves the tower at thebottom and passes to more than one externally heated vertical tube, downover the inner surface of which the acid flows in the form of a thinlayer or film. These tubes are arranged so that simultaneous parallelflow of acid takes place through 'the tubes. In this way a considerabledegree of denitration of the residual acid is effected, nitric acid andwater vapor being evolved and passing up into the dehydrating tower. Ifa sufficiently high temperature ofheating medium is used for thevertical tubes, the denitration will be complete. We consider itdesirable, however, to use lower pressure steam as the heating mediumand to remove the last trace of nitric acid from the residual acid bypassing this from the tubes through a second denitrating fractionatingtower, from which, also, the vapors evolved may pass to the bottom ofthe dehydrating tower.

In order to describe our invention more clearly, reference is made tothe accompanying drawings, which illustrate suitable equipment forcarrying out the process thereof. It is to be understood, however, thatthis is done solely by way of illustration and is not to be regarded asa limitation upon the scope of our invention. Similar numerals will beemployed throughout the several figures of the drawings to denotecorresponding parts.

Referring generally to the drawings, Figure l is a diagrammatic sketchof the apparatus of our invention including dehydrating tower, verticalevaporating tubes and denitrating tower. Figure 1A is the plan of thenotched weir. It serves to promote uniform distribution of acid to thetubes. Figure 2 is a similar sketch of a different embodiment includingthe dehydrating tower and the tubes in the absence of the denitratingtower.

Referring in greater detail to the various figures of the drawings, inFigure l, dilute nitric acid together with sulfuric acid or otherdehydrating agent is fed in at the top of the dehydrating tower 2through inlet I. As the mixture flows downward and is met by theascending hot gases, strong nitric acid vapors are given off at the topof the tower. These can pass through 3 to the usual bleaching andcondensing equipment. From the dehydrating tower the acid flows inparallel streams through weirs 4 and 5 to distributing boxes 6 and 'lwhere it reaches a level uniform with the tops of the emergent verticaltubes 8. The acid then flows down over the inner surface of these tubesin the form of a thin film. The tubes are surrounded by individualheating jackets 9, preferably containing steam at a pressure of around150 lbs. The vapors of nitric acid and water pass up through 4 and 5into the dehydrating tower. The residual acid flows out through ID,thence into the denitrating tower I I. This acid still contains a smallamount of nitric acid which is completely removed in H. Live steam isintroduced into H through l2 and the vapors from H- pass to the bottomof the dehydrating. tower 2 through tubes 8. Residual acid is drawn offat l3. It will be seen that the acid passes in parallel flow from thedehydrating tower 2 to the vertical tubes, and thence to denitratingtower H. Parallel fiow of vapors back to the dehydrating tower islikewise provided for. The heating agent for the tubes passes throughconduits I4, I5, l6 and H.

Figure 1A shows the design of the notched weir, such as is preferablyused at 4 and Sin Figure 1. These weirs serve to promote uniformdistribution of the acid leaving the bottom of dehydrating tower 2 tovessels 6 and l and tubes 8.

Figure 2 represents a process similar to that illustrated in Figure 1,except that'the denitration takes place entirely in the dehydratingtower 2 and'the vertical boiling tubes 8. The tubes are again surroundedby individual heating jackets 9; With no denitrating tower, a highertemperature is required in the heating jackets and preferablyhighpressuresteamis used, for example at 200 lbs; The residual acid leavingthe concentration system may vary considerably in strength, for example68 to 73% H2804, depending on the presence or absence of a denitratingtower, temperature of heating medium, etc.

In the foregoing description of our process we have used sulfuric acidas dehydrating agent, and this will be the one commonly used. We may,however, utilize other dehydrating materials, provided they are capableof flow. t will be necessary, of course, that the apparatus beconstructed of acid-resisting materials, and the dehydrating tower,evaporating tubes and denitrating tower may with advantage be made ofhigh-silicon iron alloys, for example. In the drawings two evaporatingtubes are shown adapted for parallel flow, but it will be apparent thatwe may employ any plurality of such tubes, depending on the desiredcapacity of the installation. For example, we have found four tubes tocomprise a satisfactory unit for use with one dehydrating tower.Preferably the tubes will be of relatively large diameter, for exampleeight inches. The tops of all the vertical tubes can with advantage beso constructed that the formation of a uniform film of liquid isfacilitated with serrated or notched edges, for example.

In the foregoing detailed description of our invention we have describedthe use of steam as the heating agent. It should be understood, however,that other such agents may be used in lieu of steam, for example,mercury vapor, diphenyl, diphenyl oxide and the like.

The process as described in the foregoing and the apparatus thereforoffers many advantages not present in the methods of the art. Theremovalof nitric acid from the dehydrating agent by this downflowing filmmethod has the advantage over the use of other types of boiling vesselspartly filled with acid, that a much more efficient heat exchangeresults. According to our method the liquid is in contact in finelydivided condition with the entire inner surface of the hot tubes,whereas in the case of previous types of vessels, for example horizontaltubes, the liquid has been in contact with onlythe lower half of thesurface of the vessel or tube. Thus the use of vertical boiling tubesincreases the capacity of the apparatus for a given volume ofevaporating vessels. For example, two vertical boiling tubes have thesame heat transfer capacity as four horizontal tubes of the same sizebecause of the increased utilization of the hot surface area of thetubes. Due to the parallel arrangement and the fact that the greaterpart of the tube volume is unoccupied, the vapor velocity is reduced,for a given amount of evaporation.

In this way, vapor surging and liquid acid entrainment is eliminated,with consequent increased efiiciency.

We have described in the foregoing the method and apparatus preferredfor carrying out our invention. It will be understood, however, thatmany details in operation and construction may be varied withoutdeparture from the scope of our invention. We intend to be limited,therefore, only by the following patent claims:

We-claim:

1. An" apparatus for the concentration of aqueous solutions of nitricacid, which comprises a dehydrating tower and more than one verticaltube of acid-resistant material, a jacketing means surrounding each ofsaid vertical tubes individually and adapted to contain a heating mediumout of contact with the acid solution, a distributing chamber at the'topof each of said tubes for maintaining a substantially constant flow ofthe acid solution in the form of a thin layer down. and over the innersurfaces of said tubes and means for allowing the flow of the acidsolution from the dehydrating tower to the distributing chambers for thetubes and for conveying the vapors from the tubes to said tower, saidvertical tubes being so arranged with respect to one another as topermit parallel flow of acid simultaneously down through the varioustubes.

2. The process of concentrating nitric acid, which comprises passing amixture of nitric acid and a dehydrating agent down through adehydrating tower of acid-resistant material, driving off nitric acidvapors therefrom, distributing the residual liquid admixture in asubstantially constant flow in the form of a thin layer down and overthe inner surfaces of a plurality of heated vertical tubes, supplyingheat to each of said tubes by individual heating means, permitting theparallel flow of said liquid simultaneously down through the varioustubes, driving ofi vapors from said liquid in said tubes and conveyingsaid vapors from said tubes back to said dehydrating tower, permittingthe same to ascend through said tower.

' RALPH F. PETERSON.

PHILIP G. WRIGHTSMAN.

